CN111558701B - Manufacturing method of fine-grain high-strength microalloy martensitic steel thin strip - Google Patents
Manufacturing method of fine-grain high-strength microalloy martensitic steel thin strip Download PDFInfo
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- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
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Abstract
The invention discloses a manufacturing method of a fine-grain high-strength microalloy martensitic steel ribbon, wherein molten steel obtained by smelting is cast into an as-cast ribbon with the thickness of 1.0-4.0mm by a twin-roll ribbon caster; taking out of the crystallization roller, and rapidly cooling to 100-300 ℃; cooling the roller, and then carrying out online rapid heating at the heating rate of 30-110 ℃/s until the austenitizing temperature is 800-1000 ℃ and the heat preservation time is 120-300 s; and cooling and curling the cast thin strip subjected to online heating to obtain a final steel coil, wherein the curling temperature is 100-300 ℃. The method realizes the refinement of the prior austenite crystal grains of the thin strip by utilizing the phase transition process and the pinning action force of the second phase particles, thereby refining the final structure of the thin strip and further improving the strength, the plasticity and the toughness of a thin strip product. In addition, the high density dispersed second phase formed by the reheating process further enhances the strength of the ribbon product.
Description
Technical Field
The invention belongs to the technical field of ferrous metallurgy, and particularly relates to a manufacturing method of a fine-grain high-strength microalloy martensitic steel thin strip.
Background
The thin strip continuous casting includes single strip continuous casting, double strip continuous casting, single roll thin strip continuous casting and double roll thin strip continuous casting. Among them, the twin roll strip casting technology is considered as the leading technology of the most revolutionary significance in the field of ferrous metallurgy in the 21 st century. In the twin-roll thin strip casting and rolling process, molten steel is directly cast on a water-cooled copper crystallization roll to form a thin steel strip with the thickness of about 1-5 mm, the molten steel generates a sub-rapid solidification effect, and a large number of reheating and rolling procedures required in the traditional continuous casting process are omitted, so that casting and rolling integrated production is realized. The twin-roll thin strip continuous casting technology can omit the hot rolling process of a casting blank or only has a small amount of hot rolling process, compared with the traditional continuous casting and thin slab continuous casting and rolling technology, the production line of the twin-roll thin strip continuous casting technology is greatly shortened, the corresponding equipment investment, the occupied area, the energy consumption and the like are obviously reduced, and the production period of steel products is greatly reduced. Therefore, the twin roll strip casting technology has recently become an advanced casting technology in competition with the development of various iron and steel companies in the world. The strip casting projects that have greatly affected the 20 th century and the 80 th century mainly include a stainless strip casting project of the new Nissan-Mitsubishi heavy industry, an EUROSTRIP strip casting project developed by the union of Europe, a CASRIP strip casting project developed by the American NUCOR iron and Steel company, a strip casting project developed by the Korea POSCO iron and Steel company, and the like. Domestic Bao steel group and business group have also built high-level twin-roll thin strip continuous casting industrialization line, have carried out the industrialization and tried out the production practice, the industrialization research has made an important progress. From the development process of the steel strip continuous casting technology, the pilot line in strip continuous casting is established and the industrial production of the strip continuous casting is realized in Europe, Japan and America, and the method is mainly used for producing carbon steel, stainless steel and silicon steel strips. However, only NUCOR steel companies in the united states have achieved commercialization of the strip casting technology and achieved good benefits, but have been mainly used for the production of low carbon steel and low carbon microalloyed steel, and have sold the CASTRIP technology to Tyasa steel companies in mexico and the sand steel group in china.
The martensitic steel is the highest strength steel type in the current commercial advanced high-strength steel, the tensile strength of the martensitic steel is usually 900-1700MPa, and the strength of some maraging steel is as high as 2 GPa. Martensitic steels are of great importance in ferrous materials, particularly in structural steel materials. The low-carbon microalloy martensitic steel has great advantages and development potential by comprehensively considering factors such as the cost, the process performance and the like of steel. Currently, the mainstream production methods of martensitic steel sheets include a hot rolling method, a cold rolling method, and a quenching method after forming. However, the methods have the defects of complex process flow, high energy consumption, long production period and the like. How to find an efficient, energy-saving and economic method for producing a martensitic steel plate is one of the directions of efforts of metallurgists. In addition, the problem of toughness reduction caused by strength increase is a bottleneck restricting the development of martensitic steel, and is generally concerned by iron and steel material researchers at home and abroad. Grain refinement is the only method capable of improving both the strength and the plasticity of steel, so in order to further improve the mechanical properties of martensitic steel, material researchers have conducted a great deal of research work in the preparation of ultra-fine grained martensitic steel, and developed numerous grain refinement methods, such as cyclic heat treatment, rapid heating, thermomechanical heat treatment, equal channel extrusion, large plastic deformation, and the like. However, these grain refining methods are based on the conventional hot rolled sheet or cold rolled sheet for further processing, and the process from the molten steel to the final ultra-fine grained martensitic steel sheet has the disadvantages of tedious process, low production efficiency, high production cost, and the like.
The invention patent with the application number of 201310398282.0 discloses a cold-rolled martensitic steel with weather resistance and a manufacturing method thereof, and the method comprises the steps of smelting, continuous casting, reheating, hot rolling, cold rolling, annealing and flattening, wherein the annealing adopts a continuous annealing mode of air cooling and overaging to produce the cold-rolled martensitic steel with weather resistance and the steel with the thickness of 1.0-1.4 mmThe yield strength of the plate reaches over 1000MPa, the tensile strength is over 1100MPa, the elongation is over 5 percent, and continuous annealing equipment with water quenching capacity is not needed. The invention patent with application number 201510360868.7 discloses martensite hot-rolled wide strip steel with tensile strength of 1500MPa and a production method thereof, molten steel is smelted according to designed components, and the molten steel is processed by a traditional continuous casting machine to obtain a plate blank; heating the plate blank to 1240-1300 ℃, descaling by high-pressure water, rolling under control, cooling under control, and coiling to obtain the final hot-rolled strip steel, wherein the produced hot-rolled coil has yield strength>1000MPa, tensile strength>1500MPa and elongation not less than 10%. The invention patent with application number 200780045534.4 discloses a cold-rolled steel sheet with high yield ratio and excellent weather resistance and a manufacturing method thereof, wherein a continuous casting slab is hot-rolled at a finish rolling temperature of 850 ℃ to 950 ℃; followed by cooling at a cooling rate of 20 ℃ to 40 ℃ per second; then coiling at 500-650 ℃; the steel coil is heated to 500 ℃ to A1Continuously annealing in the temperature range of the transformation point. The yield strength of the steel plate produced by the technology is more than 850 MPa. Compared with the emerging twin-roll thin strip continuous casting process, the three patents produce the martensite steel strip through the traditional continuous casting and rolling, have the defects of complicated production process, low production efficiency, high energy consumption and the like, and can not obtain the fine-grained martensite steel.
The invention patent with the application number of 201210193609.6 discloses an ultra-fine grain martensite steel plate and a preparation method thereof, and the ultra-fine grain martensite structure with the prior austenite grain size of less than 5 microns is obtained after the traditional plate blank continuous casting, controlled rolling and heat preservation quenching at 880-900 ℃. However, the method also has the defects of complicated production process, low production efficiency, high energy consumption and the like.
Application number 200910048141.X discloses a weathering steel and a manufacturing method thereof, the weathering steel is manufactured by a thin strip continuous casting process, and the manufacturing process comprises the following steps: deep desulfurization of molten steel; carrying out top-bottom combined blowing on the converter; RH vacuum circulation degassing process, and calcium treatment; continuously casting the thin strip by the double rollers, wherein the molten steel temperature of a molten pool is not lower than 1535 ℃; rolling is controlled, and the reduction rate is not less than 30%; controlling cooling at a cooling speed of 30-50 ℃/s; and (4) coiling at the coiling temperature of 600-650 ℃. Finally, the weathering steel with acicular ferrite structure can be obtained. However, the weathering steel does not have the characteristic of fine crystal structure, the tensile strength does not exceed 800MPa, and the application in the field of application requirements of high-strength steel is limited.
The invention patent with application number 201810512844.2 discloses a method for preparing high-strength and high-toughness martensitic steel through thin strip casting and aging processes, wherein molten steel with qualified components flows into a molten pool of a double-roll thin strip continuous casting machine, solidification and extrusion are started to obtain a thin strip with a certain thickness, then online hot rolling and cooling are carried out, then head and tail are cut off, strip steel is coiled and uncoiled, and finally the martensitic steel thin strip with the tensile strength of more than 1200MPa is obtained. However, the method requires re-uncoiling of the steel coil and then aging treatment to obtain the final product, which reduces the production efficiency of the strip continuous casting process, and the method cannot obtain the ultra-fine grained martensitic steel strip.
Disclosure of Invention
In order to solve the problems in the prior art, the invention aims to provide a method for manufacturing a fine-grain high-strength microalloy martensitic steel thin strip, which realizes the ultra-fining of the prior austenitic grains of the thin strip by utilizing the phase change process and the pinning force of second-phase particles through reasonable steel type components and thin strip continuous casting process design, thereby refining the final structure of the thin strip, simultaneously improving the strength and the plasticity of a thin strip product, and improving the toughness. In addition, by utilizing a second phase strengthening mechanism, the strength of the high-density dispersion-distributed second phase ribbon product formed in the reheating process is improved. Finally, a martensitic thin steel strip product with excellent comprehensive mechanical properties is obtained. The method can directly obtain the final martensite steel strip product from the molten steel, and has the advantages of short flow, high production efficiency, low energy consumption, environmental friendliness and the like.
In order to achieve the purpose, the invention adopts the following technical scheme:
a manufacturing method of a fine-grain high-strength microalloy martensitic steel thin strip comprises the following steps:
1) smelting of molten steel
Smelting to obtain molten steel, wherein the molten steel comprises the following chemical components in percentage by mass: c: 0.18 to 0.26%, Si: 0.10 to 0.50%, Mn: 1.0-3.0%, Ti: 0.01-0.08% or Cu: 0.4-1.0%, Mo: 0.05 to 0.50%, Nb: 0.03-0.1%, S: less than or equal to 0.006 percent, P: less than or equal to 0.02 percent, N: less than or equal to 0.007 percent, and the balance of Fe and inevitable impurities;
2) strip casting
Casting the molten steel into an as-cast thin strip with the thickness of 1.0-4.0mm by using a double-roller thin strip continuous casting machine;
3) out-of-roll cooling
Rapidly cooling the cast thin strip to 100-300 ℃ after the cast thin strip is taken out of the crystallization roller, wherein the cooling rate of the cast thin strip is not lower than 50 ℃/s;
4) on-line heating
Rapidly heating the cast thin strip after being taken out of the roll and cooled on line, wherein the heating rate is 30-110 ℃/s, the heating is carried out until the austenitizing temperature is 800-1000 ℃, and the heat preservation time is 120-300 s;
5) cooling and curling
And cooling and curling the cast thin strip subjected to online heating to obtain a final steel coil, wherein the cooling rate is not lower than 60 ℃/s, and the curling temperature is 100-300 ℃.
Further, in the step 1), the smelting mode is an electric furnace or converter steelmaking method, and the molten steel is further refined by combining vacuum degassing refining with ladle refining.
Further, in the step 2), the superheat degree of the molten steel is 80-150 ℃.
Further, in the step 2), the thickness of the as-cast thin strip is 1.5-3 mm.
Further, in the step 2), the casting speed of the twin-roll strip caster is 60-100 m/min.
Further, in the step 3), the casting-state thin film is taken out of the crystallization roller and then is cooled to 850-; and then carrying out on-line hot rolling, wherein the hot rolling reduction rate is not higher than 20%, the hot-rolled cast strip is cooled to 100-300 ℃ again, the cooling rate is 60-150 ℃/s, and the cooling mode adopts a high-pressure water nozzle for cooling.
Further, in the step 4), the online heating rate is 40-80 ℃/s, the heating is carried out until the austenitizing temperature is 850-950 ℃, and the heat preservation time is 150-250 s.
Further, in the step 5), the cooling rate is 60-150 ℃/s, and a high-pressure water nozzle is adopted for cooling in a cooling mode; the curling temperature is 200-250 ℃.
The technical concept of the invention is as follows:
1) after the cast martensite steel strip is taken out of the crystallization roller, the high-pressure inert gas is utilized to quickly and uniformly cool the strip, and the growth and coarsening of austenite grains of the high-temperature cast strip can be inhibited.
2) After the martensitic steel thin strip is hot-rolled, the martensitic steel thin strip is rapidly cooled to a temperature below the martensite phase transformation point by adopting a high-pressure water spraying mode, so that the super-cooled austenite generates non-diffusible shear phase transformation to form a martensite structure, the martensite structure has a large amount of dislocations, and the dislocations can be used as positions for forming new phases, thereby being beneficial to improving the nucleation rate of austenite when the thin strip is heated to austenitizing on line and being beneficial to refining austenite grains.
3) The cast thin strip can form larger superheat degree when being rapidly heated to austenitizing temperature at higher on-line heating rate, and the higher superheat degree can directly realize the nucleation rate of austenite so as to refine austenite grains.
4) In the online heating and heat preservation process, solute elements dissolved in the martensitic thin steel strip can be precipitated in a fine second phase and are uniformly distributed on a matrix, and for example, Nb, Ti and Mo and C, N and other elements can form corresponding carbonitride second phase particles; on one hand, the second phase particles can be pinned at the austenite grain boundary to inhibit the movement of the grain boundary, so that the austenite grains are refined; on the other hand, the second phase particles produce a precipitation strengthening effect, and improve the strength of the steel strip.
5) Generally, too high a Cu content in steel leads to hot embrittlement, which results in product scrap. However, twin roll strip casting is characterized by rapid solidification and high casting speed, and the occurrence of hot shortness is greatly reduced. Therefore, the maximum allowable value of the Cu content of the steel grade applied to the twin-roll strip casting is larger than that of the traditional continuous casting product, and the high copper content of the steel strip provided by the invention is expected to further improve the atmospheric corrosion resistance of the steel strip.
The component functions and limitations of the fine-grain high-strength microalloy martensite steel related by the invention are explained as follows:
c: c is a solid-solution strengthening element having a very good strengthening effect, and is also an element essential for transformation of austenite into a martensite phase. The C content is too high to be beneficial to the welding performance of steel, so the C content of the invention is not higher than 0.26%; however, when the C content is too low, it is not favorable for forming martensite structure and carbide, and even if the martensite structure is formed, the degree of lattice distortion is not high enough, which is not favorable for improving the strength, so that the C content of the present invention is not less than 0.18%.
Si: si has the effect of solid solution strengthening, meanwhile, the molten steel can be deoxidized and the purity of the molten steel can be improved by adding a proper amount of Si, but the surface quality of the martensitic steel strip is influenced by excessively high Si content, so that the Si content is controlled to be 0.10-0.50%.
Mn: mn is the most effective element for improving the strength and the toughness of steel, and the twin-roll thin strip continuous casting technology has the characteristic of sub-rapid solidification and can obviously reduce the segregation of Mn in an as-cast thin strip. Therefore, the Mn content of the martensitic steel produced by adopting the twin-roll thin strip continuous casting technology can be higher than that of the martensitic steel produced by the traditional continuous casting technology, the Mn content of the martensitic steel is higher than 1.0%, but the defects of difficult smelting, high alloy cost and the like caused by excessively high Mn content are overcome, and therefore the Mn content of the martensitic steel is limited to 1.0-3.0%.
Ti: ti has low solid solubility in steel and is a strong carbide forming element, and a small amount of Ti can form stable and fine titanium nitride second phase particles under high temperature conditions, thereby inhibiting the enlargement of crystal grains during austenitizing and achieving the effect of refining the crystal grains. When the content of Ti is too high, the size of titanium nitride particles becomes coarse, and the pinning force of grain boundaries is weakened, so that the particles cannot be refined, and the particles become harmful inclusions in steel, and the performance of a martensitic steel strip is deteriorated. Therefore, the amount of Ti in the steel must be strictly controlled to avoid the precipitation of coarse titanium nitride particles. On the other hand, the cost of the steel is increased due to the excessively high Ti content, so that the Ti content is controlled to be 0.01-0.08%; or Cu: the Cu element has the function of improving the atmospheric corrosion resistance of steel and is generally used in weathering resistant steel. In the traditional continuous casting process, the Cu content in steel is too high, so that the hot brittleness phenomenon can be caused, and products are scrapped, and the Cu content of steel produced by the traditional continuous casting mode is lower than 0.55 percent. But the thin strip continuous casting has the characteristics of rapid solidification and high continuous casting speed, and the hot brittleness phenomenon is greatly reduced. Therefore, the Cu content of the steel applied to the twin-roll strip casting can be higher, and the Cu content in the steel is controlled to be 0.40-1.0%.
Mo: mo exists in a steel matrix mainly in the form of solid solution and can play a role of solid solution strengthening. In addition, Mo element dissolved in steel is likely to be segregated in grain boundaries, and generates a dragging effect, thereby inhibiting the movement of austenite grains and further refining the austenite grains. However, the excessively high Mo content increases the cost of steel, so that the Mo content of the present invention is controlled to 0.05 to 0.50%.
Nb: nb atoms which are dissolved in a matrix can refine austenite grains through the dragging effect of solute atoms, and can also form niobium carbonitride second-phase grains, and the Nb atoms are precipitated from the matrix to generate the pinning force of austenite grain boundaries, so that the austenite grains are prevented from growing and refined. In addition, the addition of Nb can also improve the hardenability of martensitic steel. The Nb content is controlled to be 0.03-0.1%.
S: s is a harmful element in steel. The invention is very unfavorable to the toughness and the stamping property of the steel strip, and is easy to cause the anisotropy of the mechanical property of the steel strip, the lower the content of the sulfur in the steel is, the better the content is, the comprehensive consideration of the prior steelmaking level and economic factors, and the invention controls the content of the S to be below 0.006 percent.
P: p has the effect of improving the weather resistance of the martensitic steel, but the excessively high P content is not beneficial to the stamping performance, the welding performance, the low-temperature toughness and the like of the martensitic steel plate, and the invention focuses more on the mechanical property of the martensitic steel plate, so that the P content is not higher than 0.02 percent.
N: n is easy to combine with alloy elements in steel to precipitate carbonitride, and when the content of N is too high, the N is easy to form coarse nitride with the alloy elements in the steel, thereby having adverse effect on the plasticity and fatigue property of the steel. A small amount of N is beneficial to separating out fine carbonitride second phase particles and improving the strength of steel. Therefore, in the invention, N is controlled at a lower level, and the content is not higher than 0.007%.
The invention has the beneficial effects that:
1) compared with the traditional process for producing the ultra-fine grain martensite strip, the method has the advantages of short flow, high production efficiency, low energy consumption, environmental friendliness and the like.
2) Compared with the existing twin-roll thin strip continuous casting process, the twin-roll thin strip continuous casting process related by the invention can be used without hot rolling or with relatively low hot rolling reduction rate, thereby reducing the energy consumption in the process and reducing the loss of the hot roll, thereby reducing the production cost of ton steel. The fine-grained martensite steel coil is directly obtained by on-line heating, reheating after the steel coil is cooled to normal temperature is avoided, energy consumption is reduced, and production efficiency is improved.
3) The double-roller thin-strip continuous casting online heating process is proposed for the first time, through reasonable steel type components and thin-strip continuous casting process design, the thin-strip online heating process and the austenitizing heat preservation stage, the phase transformation process and the pinning action force of second-phase particles are utilized to realize the superfine of thin-strip original austenite grains, so that the final structure of the martensitic steel thin strip is refined, the strength and the plasticity of a thin-strip product can be improved at the same time, and the toughness is improved accordingly.
4) Through reasonable steel grade components and strip continuous casting process design, second-phase particles which are in high-density dispersion distribution are formed in the online heating process, and the strength of a strip product is improved. Finally, the martensitic thin steel strip product with excellent comprehensive mechanical property and even excellent atmospheric corrosion resistance is obtained.
Drawings
FIG. 1 is a schematic process flow diagram of a twin roll strip caster set of the present invention;
FIG. 2 is a side view of a crystallization roller and a flow distribution nozzle of the present invention;
FIG. 3 is a prior austenite grain morphology diagram of example 2 of the present invention.
FIG. 4 is a metallographic structure chart of example 11 of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other that a person of ordinary skill in the art would obtain without inventive effort based on the embodiments of the present invention
The embodiments are all within the protection scope of the present invention.
Referring to fig. 1, there is shown a process flow of the present invention:
molten steel in a ladle 1 enters a tundish 3 through a long nozzle 2, the superheat degree of the molten steel in the tundish is 100 ℃, the molten steel enters a molten pool 5 consisting of crystallizing rollers 7a and 7b and side sealing plates 6a and 6b through a flow distribution nozzle 4, and cast-state thin strips 10 with the thickness of 1.0-4.0mm are cast by the molten steel through a double-roller thin strip continuous casting machine. Wherein the width of the crystallization roller is 1500mm, the diameter of the crystallization roller is 800mm, and the casting speed of the casting machine is 90 m/min. Wire roller brushes 8a and 8b with the same width as the crystallizing rollers are uniformly arranged outside the two crystallizing rollers 7a and 7b, and the material copper alloy of the wire roller brushes 8a and 8b is used for brushing off redundant oxidation deposition films generated on the surfaces of the crystallizing rollers 7a and 7b in the continuous casting process. The area from the flow distributing nozzle 4 to the area before hot rolling is a closed space 14 which is filled with inert gas, and the inert gas is nitrogen, so that the high-temperature oxidation of the thin strip can be prevented.
After the cast-state thin strip 10 comes out of the crystallization rollers 7a and 7b, high-pressure air cooling nozzles 9 are uniformly arranged on two sides of the wide surface of the cast-state thin strip 10 along the width and the casting direction of the cast strip, and the length of the whole air cooling section is 0.3 m; the inert gas is used as the nozzle gas and helium gas, so that the oxidation of the surface of the thin strip in the cooling process can be avoided, the inert gas sprayed by the high-pressure gas cooling nozzle 9 enables the cast thin strip to be rapidly and uniformly cooled to 850 ℃ and 1100 ℃, and the cooling rate of the thin strip is not lower than 50 ℃/s. The as-cast strip 10 after exiting the crystallization rolls is fed through fan guides 11, transport rolls 12 and pinch rolls 13 into a single pass hot rolling mill 15.
And (3) carrying out a small amount of online hot rolling or no rolling by adopting a single-pass hot rolling mill 15, wherein the rolling reduction rate is 0-20%. And (3) carrying out laminar cooling on the rolled thin strip by adopting a No. 1 high-pressure water nozzle 16, wherein the cooling rate is not lower than 60 ℃/s, and directly cooling the thin strip to 200 ℃. And further carrying out on-line rapid heating on the cooled thin strip after rolling by adopting a high-frequency induction continuous heating unit 17, wherein the heating rate is 30-110 ℃/s, the thin strip is heated to the austenitizing temperature of 800-1000 ℃, and the heat preservation time is 180 s. And carrying out laminar cooling on the thin strip at the heat preservation outlet by adopting a No. 2 high-pressure water nozzle 18, wherein the cooling rate is not lower than 60 ℃/s, and directly cooling the thin strip to the curling temperature of 200 ℃.
The strip is cut by a flying shear 19 and then wound into a final coil by a powerful winder 21 under the guide of a pinch roll 20, and the coil is naturally cooled to room temperature.
The martensitic molten steel of the embodiments 1 to 7 and the comparative example 1 of the invention is obtained by electric furnace smelting, external vacuum degassing refining and ladle refining, and the specific chemical components are shown in table 1. The thickness of the cast strip, the cooling rate of the cast strip, the rolling temperature, the rolling reduction, the heating rate of the hot rolled strip, the austenitizing temperature, the austenite grain size at the heat-retaining outlet, and the tensile properties of the thin steel coil after naturally cooling to room temperature, which correspond to the examples and comparative examples, are shown in Table 2.
As can be seen from the examples 1-7 in Table 2, the Ti-containing fine-grained high-strength microalloy martensite steel strip has the yield strength of more than 1000MPa, the tensile strength of more than 1400MPa, the elongation of more than 17 percent and excellent comprehensive mechanical properties. In comparative example 1, since the on-line heat treatment was not performed, austenite grains were coarse, and the strength and toughness were poor.
The molten steels of examples 8 to 14 and comparative example 2 of the present invention were obtained by electric furnace smelting, external vacuum degassing refining in combination with ladle refining, and the specific chemical compositions are shown in table 3. The thickness of the cast strip, the cooling rate of the cast strip, the rolling temperature, the rolling reduction, the heating rate of the hot rolled strip, the austenitizing temperature, the austenite grain size at the heat-retaining outlet, and the tensile properties of the thin steel coil after naturally cooling to room temperature, which correspond to examples and comparative examples, are shown in Table 4.
As can be seen from the examples 8-14 in Table 4, the Cu-containing fine-grain high-strength microalloy martensite steel strip has the yield strength of more than 900MPa, the tensile strength of more than 1200MPa, the elongation of more than 14 percent and excellent comprehensive mechanical properties. In comparative example 2, since the on-line heat treatment was not performed, austenite grains were coarse, and the strength and toughness were poor.
The weather resistance test conditions of the Cu-containing fine-grain high-strength microalloy martensite steel strip are as follows: the steel belt is subjected to a salt spray test for one month by using a 5% NaCl solution as a spraying solution at the temperature of 35 ℃, and the spraying amount is 0.8-1.5 ml/h. The results of the salt spray corrosion test of the steel strip are shown in table 5. The atmospheric corrosion resistance of the Cu-containing fine-grain high-strength microalloy martensite steel strip is obviously better than that of straight carbon steel Q235.
TABLE 5 salt spray Corrosion test results for thin steel strips made in examples 8-14
| Amount of corrosion, mm/a | Relative corrosion rate,% of | |
| Example 8 | 0.489 | 57.33 |
| Example 9 | 0.526 | 61.66 |
| Example 10 | 0.425 | 49.82 |
| Example 11 | 0.472 | 55.33 |
| Example 12 | 0.501 | 58.73 |
| Example 13 | 0.389 | 45.60 |
| Example 14 | 0.445 | 52.17 |
| Q235 | 0.853 | 100 |
Claims (8)
1. A method for manufacturing a thin strip of fine-grained high-strength microalloyed martensitic steel is characterized by comprising the following steps:
1) smelting of molten steel
Smelting to obtain molten steel, wherein the molten steel comprises the following chemical components in percentage by mass: c: 0.18 to 0.26%, Si: 0.10 to 0.50%, Mn: 1.0-3.0%%, Ti: 0.01-0.08% or Cu: 0.4-1.0%, Mo: 0.05 to 0.50%, Nb: 0.03-0.1%, S: less than or equal to 0.006 percent, P: less than or equal to 0.02 percent, N: less than or equal to 0.007 percent, and the balance of Fe and inevitable impurities;
2) strip casting
Casting the molten steel into an as-cast thin strip with the thickness of 1.0-4.0mm by using a double-roller thin strip continuous casting machine;
3) out-of-roll cooling
Rapidly cooling the cast thin strip to 100-300 ℃ after the cast thin strip is taken out of the crystallization roller, wherein the cooling rate of the cast thin strip is not lower than 50 ℃/s;
4) on-line heating
Rapidly heating the cast thin strip after being taken out of the roll and cooled on line, wherein the heating rate is 30-110 ℃/s, the heating is carried out until the austenitizing temperature is 800-1000 ℃, and the heat preservation time is 120-300 s;
5) cooling and curling
And cooling and curling the cast thin strip subjected to online heating to obtain a final steel coil, wherein the cooling rate is not lower than 60 ℃/s, and the curling temperature is 100-300 ℃.
2. The method of making a thin strip of fine-grained high-strength microalloyed martensitic steel as claimed in claim 1 wherein: in the step 1), the smelting mode is an electric furnace or converter steelmaking method, and the molten steel is further refined by combining vacuum degassing refining with ladle refining.
3. The method of making a thin strip of fine-grained high-strength microalloyed martensitic steel as claimed in claim 1 wherein: in the step 2), the superheat degree of the molten steel is 80-150 ℃.
4. The method of making a thin strip of fine-grained high-strength microalloyed martensitic steel as claimed in claim 1 wherein: in the step 2), the thickness of the cast thin strip is 1.5-3 mm.
5. The method of making a thin strip of fine-grained high-strength microalloyed martensitic steel as claimed in claim 1 wherein: in the step 2), the casting speed of the twin-roll strip caster is 60-100 m/min.
6. The method of making a thin strip of fine-grained high-strength microalloyed martensitic steel as claimed in claim 1 wherein: in the step 3), the cast thin film is taken out of the crystallization roller and then is cooled to 850-1100 ℃, the cooling rate is 50-100 ℃/s, the cooling mode adopts a high-pressure air cooling nozzle for cooling, and the nozzle gas is nitrogen, helium or argon; and then carrying out on-line hot rolling, wherein the hot rolling reduction rate is not higher than 20%, the hot-rolled cast strip is cooled to 100-300 ℃ again, the cooling rate is 60-150 ℃/s, and the cooling mode adopts a high-pressure water nozzle for cooling.
7. The method of making thin strip of fine-grained high-strength microalloyed martensitic steel as claimed in claim 1 or 6, characterized in that: in the step 5), the online heating rate is 40-80 ℃/s, the heating is carried out until the austenitizing temperature is 850-950 ℃, and the heat preservation time is 150-250 s.
8. The method of making thin strip of fine-grained high-strength microalloyed martensitic steel as claimed in claim 1 or 6, characterized in that: in the step 5), the cooling rate is 60-150 ℃/s, and a high-pressure water nozzle is adopted for cooling in a cooling mode; the curling temperature is 200-250 ℃.
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